Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A typical cellular wireless network includes a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
In an example arrangement, the network infrastructure may include one or more packet data network gateways (PGWs) or similar components that provide connectivity with a packet-switched network so as to support various communication services. For instance, the infrastructure may include gateways that support general packet-data communications, such as general web browsing, file transfer, and the like, and/or packet-based real-time media communications such as voice over Internet Protocol (VoIP) and streaming media for instance.
A representative PGW may sit as a node on a wireless service provider's private packet-switched network and may thus provide connectivity with various application servers and other entities on that private network, and with other such entities accessible through a connection between the service provider's network and one or more other networks such as the public Internet. By way of example, such a PGW may provide connectivity with an Internet Multimedia Subsystem (IMS) platform or other session server that supports VoIP calling and/or other such media services.
When a UE first enters into the coverage of cellular wireless network, the UE engages in a process of registering or “attaching” with the network, which may trigger setup of various communication channels for the UE and/or reservation of various communication resources for the UE. For instance, upon first detecting coverage of a base station, the UE may transmit an attach request message to the base station, which the base station may forward to a network controller such as a mobility management entity (MME). Upon authenticating and authorizing the UE, the network controller may then engage in further signaling with the base station and with a serving gateway (SGW), which may in turn engage in signaling with a PGW, ultimately resulting in setup of one or more bearer connections or “bearers” each extending, via the base station, between the UE and the PGW, through which the UE can then engage in packet-data communication via the PGW.
In practice, a network such as this may initially establish for a UE one or more default bearers to enable the UE to engage in certain basic communications, with each default bearer having a respective quality of service level. By way of example, the network may initially establish for the UE a default Internet bearer with a best-efforts quality of service level, for use by the UE to engage in general Internet communications such as web browsing, e-mail messaging, and the like. Further, if the UE subscribes to VoIP service or another such service that would be served by an IMS, the network may initially establish for the UE a default IMS signaling bearer with a medium quality of service level, for use by the UE to engage in session setup signaling (such as Session Initiation Protocol (SIP) signaling) with the IMS to facilitate setup of VoIP calls or the like.
Further, as the UE is served by the network, the network may establish for the UE additional bearers as needed. For example, if the UE has an IMS signaling bearer and the UE engages in signaling over that bearer with an IMS to set up a packet-based real-time media session such as a VoIP call, the network may then establish for the UE a dedicated IMS bearer with a high quality of service level, for carrying media content of the session, such as VoIP voice packets, to and from the UE. Once the dedicated IMS bearer is established, the UE may then send and receive media content of the session over that dedicated IMS bearer.
By way of example, during a VoIP call, a UE may send and receive voice frames to a remote network entity using the dedicated IMS bearer. To communicate with the remote network entity, the UE may send and receive the voice frames to a base station serving the UE over an air interface. Each of the voice frames may be of a particular length and correspond to a particular portion of a voice signal. For instance, in an example implementation, each voice frame may correspond to a 20-millisecond portion of speech. Additionally, each voice frame is typically packetized into a single data packet (e.g., a Real-time Transport Protocol (RTP) packet) along with various associated headers, and each such data packet may be scheduled for transmission individually. In order to maintain a continuous flow of the voice frames during the VoIP call and keep the voice delivery in approximately real-time, the voice frames may be communicated at largely the rate defined by the voice frame length. As an example, where each voice frame corresponds to 20 ms, the voice frames may be communicated at largely a rate of approximately one voice frame per 20 ms.
Further, in some wireless communication systems, when data that is transmitted by a transmitting entity to a receiving entity is not received by the receiving entity, or is received by the receiving entity with one or more errors, the data may be retransmitted. The retransmission of data could occur either automatically or in response to feedback from the receiving entity. For example, in Long Term Evolution (LTE) systems, and in other systems, a Hybrid Automatic Repeat Request (HARQ) procedure is used. In the HARQ approach, after a transmitting entity has transmitted a block of data, the transmitting entity waits to receive an HARQ response from the receiving entity. If the transmitting entity receives a positive acknowledgement (ACK) as the HARQ response, then no-retransmission is needed and the transmitting entity can transmit additional data. If the transmitting entity receives a negative acknowledgement (NACK) as the HARQ response, then the transmitting entity retransmits the data. The transmitting entity may also retransmit the data if the transmitting entity does not receive any HARQ response within a certain period of time.
This retransmission approach can allow data to be successfully transmitted from a transmitting entity to a receiving entity even when there is a substantial probability that the transmitted data will be received with one or more errors, for example, because of poor radio frequency (RF) conditions. Specifically, the data can be retransmitted multiple times until the data is received without errors. This retransmission approach, however, also increases latency. For example, there can be a period of delay between when the transmitting entity transmits data and when the transmitting entity receives a NACK response from the receiving entity and another period of delay between when the transmitting entity receives the NACK response and when the transmitting entity begins retransmitting the data.
During a VoIP call, there may be an overall acceptable delay tolerance (e.g., 200 ms) between a time when a voice frame is spoken at one end of a conversation to a time when a data packet corresponding to the voice frame is received and decoded the other end of the conversation. A portion (e.g., 41 ms) of the overall acceptable delay tolerance may be allocated to the physical RF link between a UE and a base station. With such a delay tolerance on the RF link, a certain number of retransmissions over the air interface may be tolerable for a single voice frame, while staying within the delay tolerance for the RF link, but any more retransmissions than that may result in dropping of the voice frame.
By way of example, if the base station is configured to retransmit data packets using an HARQ procedure, each retransmission may add 8 ms of delay: 4 ms waiting for a NACK response or non-response, and an additional 4 ms waiting to retransmit the data packet. With this example, two retransmissions of a 20-ms voice frame would add 16 ms of delay, such that the total delay due to digitizing the voice frame and the retransmissions would be 36 ms, which is within the 41-ms tolerance. On the other hand, three retransmissions of a 20-ms voice frame would add 24 ms of delay, such that the total delay due to digitizing the voice frame and the retransmissions would be 44 ms, which exceeds the 41-ms delay tolerance.